A. Technical Field
This invention relates to the field of mechanical devices, including computer-controlled mechanical devices, which simulate the action of human breathing. One use of such devices is to test the operation of mechanical ventilators. In the preferred embodiment, the invention is a servo-controlled mechanical piston which simulates the mechanical properties of the human lung, chest wall and diaphragm. The invention includes the control method for the servo lung simulator which is implemented on a digital computer through software which provides the simulation and control signals for the mechanical piston.
B. Background Art
In the prior art, there were published standards for the minimum performance of mechanical ventilators. These published standards include ANSI Standard Z79.7, ISO Standard 5469.:1987, and ASTM Standard 1100. Testing of ventilators to determine whether these standards are met may be part of new prototype product evaluation, quality assurance during the manufacturing process, calibration in the field or as part of a routine service program. Ideally, any testing device for medical devices should meet or exceed the minimum performance standards. However, there are a few existing test lungs that exceed these published standards. The TTL, training and test lung manufactured by Michigan Instruments (Grand Rapids, Mich.) is probably one of the best known. This device uses two spring loaded bellows to create a two compartment mechanical lung simulation. A few discrete resistances can be modeled using fixed resistive elements that are added in the tubing of the test lung. Compliance is adjusted by changing the spring loaded force on the upper plate of the bellows. Lung pressure is measured using traditional pressure gauges on the front panel. Lung volume is measured by observing the displacement of the upper bellows plate. Michigan instruments sells a PC board and software that will collect data from the pressure transducers and using the compliance setting, which must be keyed in by the user, calculate volumes. This system is specified to have an accuracy of 5%. The system does provide a crude mechanism for providing triggering events; however, the ability to simulate a two compartment lung is lost if triggering is used. The lung simulator does not provide a reasonable method for simulating spontaneous respiration. There are other systems such as the Biotek system that include a pneumotach and are somewhat more sophisticated; however, in general they all have similar problems. Essentially there are not tools that can be used to reliably test mechanical ventilators throughout the spectrum of their capabilities.